This invention relates to an abnormal separation detecting circuit of chromatic signals which detects that whether series signals of R-Y signals and B-Y signals comprising color television chromatic signals which are transmitted according to a SECAM system are correctly separated according to a predetermined timing or not.
FIG. 1 shows one example of the construction of the chromatic signals utilized in the SECAM system and constituted by R-Y signals and B-Y signals which are transmitted alternately in each horizontal scanning period T.sub.h. Between the R-Y and B-Y signals are interposed synchronizing signal periods T.sub.s containing identification signals I.sub.1 and I.sub.2 having different forms and utilized to show that a signal to be sent next time is an R-Y signal or a B-Y signal. The identification signal period is designated by T.sub.i in FIG. 1. More particularly, at the head of an R-Y signal is added an identification signal I.sub.1 whereas an identifcation signal I.sub.2 is added to the head of each B-Y signal.
In addition to the chromatic signals, a color television receiver also receives a brightness signal Y (Y=.alpha.R+.beta.G+.gamma.B, where R, G and B represent red, green and blue color component, respectively). Based on these chromatic signal and the brightness signal are formed R, G, and B color signals which are used to display a picture image on a cathode ray tube.
On the receiving side R-Y signals and B-Y signals which are alternately sent in succession are connected into the form of parallel signals and R-Y signals and B-Y signals are separately derived out respectively by an R-Y signal receiving unit and a B-Y signal receiving unit.
FIG. 2 shows a converting and separating circuit which connects serially received R-Y signals and B-Y signals into parallel signals and to separate the R-Y signals and the B-Y signals under the control of suitable pulse signals. The converting unit is constituted by a delay circuit whereas the separating unit is constituted by two switches 14 and 16.
The delay circuit 12 operates to delay the R-Y signals and B-Y signals applied thereto by one horizontal scanning period T.sub.h (one R-Y signal period or one B-Y signal period) whereas the switches 14 and 16 are opened and closed in response to pulse signals CL.sub.1 and CL.sub.2 having opposite phases.
In the circuit shown in FIG. 2, a chromatic signal comprising serially transmitted R-Y and B-Y signals is applied to an input terminal T.sub.in. The chromatic signal is applied to the input of the delay circuit 12 and to one stationary contacts 14a and 16a of the switches 14 and 16. The signal delayed by the delay circuit 12 is applied to the other stationary contacts 14.sub.b and 16.sub.b of the switches 14 and 16 which respectively receive pulse signals CL.sub.1 and CL.sub.2 having opposite phases and vary their levels in synchronism with the switching of the horizontal scanning period T.sub.h of the chromatic signals, to transfer their movable contacts 14c and 16c between the stationary contacts 14a, 14b and 16a, 16b. Thus, during one horizontal scanning period T.sub.h, the switchs 14 and 16 are thrown to the stationary contacts 14b and 16a respectively whereas during the next horizontally scanning period to the stationary contacts 14a and 16b respectively.
In this manner, the R-Y signals and the B-Y signals which are alternately sent to the input terminal T.sub.in are supplied directly to switches 14 and 16 and indirectly supplied thereto via the delay circuit 12 which delays these signals by one horizontal scanning period T.sub.h (one R-Y signal period or one B-Y signal period) and by ON OFF controlling the two switches 14 and 16 according to a predetermined timing, only the R-Y signals can be derived out from an R-Y signal terminal T.sub.R-Y, whereas only the B-Y signals can be deribed out from a B-Y signal terminal T.sub.B-Y.
The converting and separating circuit shown in FIG. 2 operates as follows.
The converting and separating circuit is designed such that concurrently with the application of a B-Y signal upon the input terminal T.sub.in, the movable contacts 14c and 16c are thrown to the stationary contacts 14b and 16a as shown in FIG. 2. On the other hand, when an R-Y signal is applied, the movable contacts 14c and 16c are thrown to the stationary contacts 14a and 16b respectively. The timing of transfer of the switches 14 and 16 is controlled by signals CL.sub.1 and CL.sub.2 respectively. Suppose now that the levels of the pulses CL.sub.1 and CL.sub.2 vary at a predetermined timing and hence the switches 14 and 16 are transferred in a predetermined manner, whereby the movable contacts 14c and 16c are thrown to the stationary contacts 14b and 16a respectively concurrently with the application of the B-Y signal upon the input terminal T.sub.in.
At this time, the stationary contact 16a of switch 16 is supplied with the B-Y signal supplied to the input terminal T.sub.in, and this signal B-Y appears at the output terminal T.sub.B-Y via switch 16. Under these condition, the delay circuit 12 produces an R-Y signal one period before B-Y signal now being applied to the input terminal T.sub.in, and this R-Y signal appears at the output terminal T.sub.R-Y via the movable contact 14.sub.b of switch 14.
Where an R-Y signal is applied to the input terminal T.sub.in since switches 14 and 16 are transferred at the same time with the application of the R-Y signal as above described so that the movable contacts 14.sub.c and 16.sub.c are thrown to the stationary contacts 14a and 16b concurrently with the application of the R-Y signal.
At this time, the R-Y signal applied to the input terminal T.sub.in appears on the output terminal T.sub.R-Y via switch 14. On the other hand the delay circuit 12 produces a B-Y signal one horizontal scanning period before the R-Y signal now being received by the input terminal T.sub.in, and this B-Y signal appears on the B-Y signal output terminal T.sub.B-Y.
In this manner, so long as the timing of switching of signals R-Y and B-Y which are received alternately and the timing of varying the levels of the pulse signals CL.sub.1 and CL.sub.2 that control the operation of switches 14 and 16 are synchronous, signals R-Y and B-Y which are sent alternately are perfectly isolated so that signal R-Y would be produced only at the output terminal T.sub.R-Y whereas signal B-Y would be produced only at the output terminal T.sub.B-Y. Accordingly clear picture images would be displayed on the cathode ray tube.
However, should the timing shift, for example by one horizontal scanning period with respect to a prescribed timing the shape of the signal derived out from output terminals 14 and 16 would be reversed.
For this reason, in order to obtain clear picture images it is necessary to constantly supervize whether signals R-Y and B-Y are perfectly isolated or not.
To this end, as shown in FIG. 1, identification signals I.sub.1 and I.sub.2 which are to be modulated and respectively having frequencies of about 4.2 MHz and about 4.4 MHz are added to the heads of signals R-Y and B-Y respectively. Descrimination of these identification signals I.sub.1 and I.sub.2 makes it possible to known the type of the signals now being received (R-Y signal or B-Y signal) and the fact that these signals are perfectly isolated or not.
However, a prior art detection circuit utilized for this purpose utilizes a number of capacitors so that in order to fabricate it with an integrated circuit, it is necessary to provide a number of external terminals for connection of the capacitors thus making it impossible to fabricate the circuit with an integrated circuit.